Leaving it switched on, means leaving it on and not resetting it every time you want a faster sync speed.
Like it or not, some lines, especially longer ones, are subject to interference, which gets worse in the evenings. There isn't a great deal you can do about this, if you've checked everything in your home, which it seems that you have.
As more people get faster broadband connections on your exchange, increased crosstalk (essentially the signal leaking from one phone line onto another one) will unfortunately interfere with your own connection. Something else along the long path of your line may have recently started to cause additional interference. Either way, I don't think you're going to be able to do anything about it.
You are still getting a good speed for your attenuation, as Eric has said.
So basically you really have two choices:
1. Leave the router alone, don't keep resetting it. It will settle at a certain speed and stay there.
Pros: Connection stability. Don't have to keep messing with router.
Cons: Not as quick during the day when it could be quicker.
2. Keep resetting the router during the day in order to sync higher.
Pros: Faster speed during the day, assuming IP profile is updated accordingly.
Cons: Line resyncs in evening as SNR drops below usable level. Also BT's DLM may interpret resetting router as a fault and bump up your SNR margin, causing you to get even lower sync speeds overall.
Hope that helps
